The present invention relates to a stabilizing circuit for stabilizing the horizontal picture size on the display of a television receiver, in particular to a stabilizing circuit for a television receiver of a multi-scanning type which is capable of receiving various video signals having different horizontal scanning frequencies.
There has been known a television receiver of a multi-scanning type, which serves as a common receiver to receive several kinds of video signals including television broadcast signals as well as personnel computer signals.
However, in such a television receiver of multi-scanning type, it is required that horizontal picture size be stabilized. In order to achieve this, although the horizontal scanning frequency might vary within a range of 15-37 KHz, it is necessary to stabilize a horizontal amplitude and high voltage output to CRT (cathode-ray tube), thereby stabilizing the horizontal picture size.
FIG. 4 indicates an example of a conventional stabilizing circuit for stabilizing the horizontal picture size on the display of a television receiver. As shown in FIG. 4, this conventional circuit includes a horizontal osillator 10, a horizontal output stabilizing circuit 20, and a horizontal output circuit 30.
In FIG. 4, the horizontal oscillator 10 comprises a VCO (voltage-control oscillator) using a multi-vibrator. The osillating frequency of the VCO is controlled by a control voltage from a horizontal AFC (automatic frequency controller) (not shown), thus the osillating frequency of the VCO may synchronize with a horizontal synchronizing signal so as to have the same frequency as the synchronizing signal. In this way, when a video signal having a different horizontal scanning frequency is received, the control voltage from the horizontal AFC will change, therefore the frequency of pulse output from the horizontal osillator 10 will also be changed corresponding to the horizontal scanning frequency.
The horizontal output stabilizing circuit 20 has a control transistor Tr1 which is provided with a power source + B on the collector side thereof, and a transformer T which has a ratio 1:1 of primary voltage to secondary voltage. With the control of the transistor Tr1, an output pulse Vb from the secondary coil of the transformer T is maintained constant. Therefore, it is possible to maintain constant a horizontal collector pulse Va (collector voltage) of a horizontal output transistor Tr2 which is shown in the drawing and will be described in detail later.
Namely, in order that the output pulse Vb may be maintained constant, a voltage difference as compared with a reference voltage Vref is output from a differential amplifier 21. Thus, an output corresponding to this voltage difference controls the impedance of the transistor Tr1. In this way, an output Vcc corresponding to the aforesaid voltage difference is outputted at the emitter of the transistor Tr1, and then applied to the primary coil of the transformer T.
On the other hand, the horizontal output circuit 30 is a circuit which includes, as arranged in a manner shown in FIG. 1, a horizontal deflection coil LH, a horizontal output transistor Tr2, a resonant capacitor C.sub.R, a damper diode D.sub.D, and a correction capacitor Cs. By supplying a horizontal deflection current iH to the horizontal deflection coil LH, a desired horizontal deflection output may be obtained. When the transistor Tr2 is ON, the horizontal deflection current iH flows into the horizontal deflection coil LH. When the transistor Tr2 is OFF, the horizontal deflection current iH which had flowed into the coil LH will flow into the resonant capacitor C.sub.R. Afterwards, the electric charges accumulated in the resonant capacitor C.sub.R are discharged through the horizontal deflection coil LH, the back electromotive force of the coil LH causes the conducting of the damper diode D.sub.D, so that the electromagnetic energy of the coil LH is caused to feedback to the correction capacitor Cs. In this way, the horizontal deflection current iH flowing through the damper diode D.sub.D into the coil LH decreases and soon become zero, thereby obtaining a horizontal deflection output having a saw-tooth waveform.
During the horizontal blanking interval, a pulse Va occurs at the collector side of the transistor Tr2 and at the horizontal deflection coil LH, and such pulse Va is applied to the primary coil of of the transformer T.
Accordingly, an output pulse Vb occurs at the secondary coil of the transformer T. After such an output pulse Vb is smoothed through a rectifying/smoothing circuit including a diode D1 and a capacitor C1, the pulse Vb is inputted into the differential amplifier 21. With the use of a voltage difference as compared with the reference voltage Vref, the control transistor Tr1 is controlled, so that the output pulse Vb on the secondary side of the transformer T is maintained constant.
However, in the conventional stabilizing circuit illustrated in FIG. 4, when there is an increase in the horizontal scanning frequency of a horizontal cychronizing signal being input into the base of the transistor Tr2, there will be an increase in The frequency of the horizontal collector pulse (collector voltage), hence the horizontal deflection output becomes small, resulting in an unstable horizontal picture size on a television display. Accordingly, the output Vcc from the emitter of the transistor Tr1 should be increased so as to maintain constant the horizontal deflection output.
That is, when the ratio of the primary voltage to the secondary voltage of the horizontal output transformer T is 1:1, and when the output pulse Vb from the secondary coil thereof is caused to feedback under a stabilized level, the collector pulse (collector voltage) Va of the horizontal output transistor Tr2 may be expressed in the flowing equation. ##EQU1## where tr is blanking interval (constant)
ts is scanning time PA1 T is cycle PA1 f (=1/T) is horizontal scanning frequency
As is understood from the above equation (2), when the horizontal scanning frequency f changes, the output Vcc is not completely proportional to such frequency f. As a result, it is impossible to maintain constant the collector pulse (collector voltage) Va of the horizontal output transistor Tr2, hence it is difficult to stabilize the horizontal picture size on the display of a television receiver if the horizontal scanning frequency changes.